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G. L. HENDERSON SELECTED FILM VELOCITY DISTILLATION Dec. 3, 1968 8 Sheets-Sheet 8 Filed Jan. 27, 1966 :iwi sw w United States Patent 3,414,483 SELECTED FILM VELOCITY DISTILLATION George L. Henderson, Seattle, Wash., assignor of six percent to F. V. Lockman and ninety-four percent to Joseph B. Ward, Calvin B. Lake, M. F. Riback and George Henderson Continuation-in-part of application Ser. No. 233,608,

Oct. 29, 1962. This application Jan. 27, 1966, Ser.

11 Claims. (Cl. 203-10) This is a continuation-impart of -my copending United States patent application entitled, Apparatus and Method for Evaporating Brine, Ser. No. 233,608, filed Oct. 29, 1962 now abandoned.

This invention relates to the evaporation of liquids and more particularly to a process and an apparatus for the evaporation of aqueous solutions of solid materials, such as brines.

Common salt, sodium chloride, is a low cost material in many areas. In other areas, freight costs add to its price as much as or more than the original cost of the material itself. In such areas, there is sometimes available a brine, such as sea water, which might be evaporated, but in general evaporation costs have been prohibitive.

In some Seacoast areas where sunny climates exist it has been possible to effect solar evaporation economically but solar evaporation requires substantial amounts of available land and rising land costs have made solar evaporation uneconomical in areas which might otherwise be quite suitable for it. In addition, solar evaporation generally produces no usable condensate and therefore provides no readily available washing medium and permits only partial purification of the sodium chloride from other dissolved salts.

Many schemes for the evaporation of sea water have been proposed, both for the recovery of its salt content and for the recovery of its water. Of these schemes, those involving vapor compression systems are among the most economical. In a vapor compression system, the vapors evaporated from the brine are heated to a higher temperature by substantially adiabatic compression and used at the higher temperature as a source of heat for the evaporation of additional brine. In essence, such processes use compression work rather than direct addition of heat to supply the external energy required for evaporation.

While many evaporation systems of this or of a somewhat similar nature are operable, and do serve useful functions in some situations, there are problems in attempting to employ them more generally in commercial operations. With a good number of systems, the efiiciencies are simply too low with the result that unless abundant power can be made available at a very low cost, the system is simply not economically feasible. In other cases where a system has been so devised as to obtain better efficiency, other difficulties have offset that advantage. For example, some of these systems utilize apparatus which requires a capital outlay that is prohibitive in terms of the expected output of the system. And sometimes the operation of the system is such that the maintenance becomes too costly in that it requires frequent replacement of parts and/or servicing of the equipment, possibly along with frequent shutdowns of the system to perform the maintenance thereof. t

In evaporation systems wherein the liquid being evaporated is heated by indirect heat exchange With a heating fiuid, it is the general practice to maintain a relatively high temperature differential between the two uids in order to get maximum utilization of the heating surface of the heat transfer wall. Achievement of this goal, however, is more illusory than real since high temperature differentials result in uneven heating and in the develop- ICC ment of hot spots on the heat transfer surface. This, in turn, results in the deposition of salts on the heat transfer surface with consequent impairment of heat transfer through the wall, thus defeating the very purpose for which the high temperature differential was adopted.

In accordance with one embodiment of this invention there is provided a method for evaporating a brine which comprises passing said brine downwardly as a flowing film over a heat transfer wall, passing a heated fluid into contact with the opposite side of said heat transfer wall at a temperature no greater than about 15 F.-above the temperature of said flowing film, and imparting a downward velocity 4on said film at the upper heat transfer level of the heat transfer wall at least equivalent to the velocity achieved by a free falling film of said brine on a smooth vertical wall after a fall of about three feet.

It has been found that despite the relatively low temperature differential maintained in this method, the heat transfer is high because of its efiiciency. The low temperature differential prevents the formation of local hot spots on the heat transfer Walls and the high velocity of the flowing film enhances heat transfer thereto. Heat transfer to a film in turbulent flow is proportional to the velocity to the 0.8 power.

In addition, the high film velocity prevents the adherence of any precipitated salt to the heat transfer surface and instead keeps such precipitates suspended in the liquid, thereby avoiding fouling of the heat transfer surface.

In accordance with a second embodiment of this invention, there is provided an apparatus which has been found to be especially effective in evaporating the brine in the initial stages of the overall evaporative process at which stages the salt content of the brine is not excessively high. This apparatus is made up of components, most of which can be supplied quite economically and which can be constructed into an operating system with relative ease. The mode of operation of the apparatus is such that it does not require more sophisticated equipment seen in many prior art devices (e.g., high performance equipment which may have many moving parts or which must be made to particularly high specifications since it is to be subjected to high temperatures and/ or pressures or other demanding conditions). Concomitant with this is the fact that the problems of maintaining the apparatus in proper working order are simplified.

Yet it has been found that this apparatus, While having these various advantages, is able to function with high efficiency, not only in terms of relatively high heat transfer coefficients across its heat transfer surfaces, but also in terms of overall efficiency (i.e., power input per unit of fresh water recovered). This is in contnast to many of the prior .art devices where to improve the perfomance of the system it has been necessary to incorporate more sophisticated devices, which unfortunately complicate the operation of the system and detract from its overall desirability for a commercial operation.

As a generalizatiomit may be said that in its preferred embodiment the present invention achieves its ends by creating in an especially effective rrranner a proper bal- Iance of the various factors which affect the efficiency of the system, as Iwell as other practical aspects of the same. This balance has various facets. For instance, the various factors which affect heat transfer across the heat transfer surfaces are brought into proper relationship so that the overall heat transfer coeicient is high. Yet this is accomplished in such a manner that there is not imposed on the system burdens disproportionate tothe benefits of achieving these high heat transfer coefficients. (For example, the power requirements, the circulating or recycling equipment, the structural material and other equipment are so yarranged and so cooperate with one another that they are kept within pratical limits.) In the field of desalinization especially, where the operability of many different systems has long been an accepted fact, but where the economic feasibility of most .all of these systems remains as the panamount consideration, the importance of this balance throughout the system cannot be too greatly stressed.

In the present invention, it becomes practical, and in certain aspects quite desirable, to utilize vertically disposed heat transfer 'walls of a greater height than was heretofor practical. This, of course, `makes it possible to obtain greater heat transfer surface without usi-ng more ground area and without a corresponding increase in circulating and/or recycling equipment and various structural components. There is the further positive benefit that this increased height enhances heat transfer on each wall surface down which the distilland falls by virtue of the fact that the turbulence of the distilland increases as its velocity increases. On the opposite side :of each heat transfer Wall, the heat transfer is maintained at a correspondingly high level by keeping the thickness of the condensate film within proper limits, this being very effectively accomplished by means of traps provided on the condensate surface at properly spaced vertical intervals.

Another facet of the present invention is that a vapor collecting manifold is provided at an especially advantageous location at the side edges of the heat exchange walls. The evaporating process is so controlled and this manifold is so arranged that it can withdraw the vapor directly from the vaporizing chambers which are defined by adjacent heat transfer Walls, without resorting to a separate flashing or settling chamber from which vapor can be collected.

Further, with the evaporating system of the present invention, the construction of the evaporating .apparatus in a preferred embodiment involves essentially the erection =of a plurality of vertically disposed, parallel, moderately spaced heat transfer \walls, arranged in ,a manner to create alternate evaporating and condensing chambers, Such an evaporating unit can be made essentially in the form of a relatively simple box-like structure, with the component parts thus being able to be supplied economically and be easily erected.

My invention may be more readily understood by reference to the drawings of which:

FIGURE 1 is a semi-schematic flow sheet showing the overall process of this invention;

FIGURE 2 is a semi-schematic flow sheet showing the multi-stage atmospheric evaporation step of the overall process;

FIGURE 3 is a semi-schematic cross-sectional view showing the structure of the apparatus used in a single stage of the atmospheric evaporation;

FIGURE 4 is a cross-sectional view through plane 4-4 of FIGURE 3;

FIGURE 5 is a fragmentary side elevational view of an evaporating unit of a second embodiment of the present invention;

FIGURE 6 is an isometric view of the apparatus of FIGURE 5, With portions thereof being broken away;

FIGURE 7 is a framentary sectional view taken on line 7 7 of FIGURE 5;

FIGURE 8 is a fragmentary horizontal sectional vielw taken on line 8 8 of FIGURE 7;

FIGURE 9 is a fragmentary horizontal sectional view taken on line 9-9 lof FIGURE 7;

FIGURE l0 is a fragmentary vertical sectional view taken on line 10-10 of FIGURE 5;

FIGURE 11 is a fragmentary horizontal sectional view taken on line 11-11 015 FIGURE l0;

FIGURE 12 is a sectional view, drawn to an enlarged scale and taken on line 12-12 of FIGURE 1l;

FIGURE 13 is a view drawn to an enlarged scale and detailing generally that portion `of the apparatus circled at 13 in FIGURE 8;

FIGURE 14 is a sectional view drawn to an enlarged soale and detailing that portion of the apparatus indicated at 14-14 of FIGURE 15, and

FIGURE l5 is a fragmentary isometric view illustrating the lower front portion of the apparatus shown in FIG- URE 6.

FIGURE 16 is a rst graph illustrating certain heat transfer `characteristics of the present invention;

FIGURE 17 i-s a second graph illustrating various perfonmance characteristics of a specific embodiment -of the invention, and

FIGURE 18 is Ia third graph illustrating certain localized heat transfer characteristics across a heat transfer wall.

Referring specifically to FIGURES 1 and 2, sea water is drawn into the system through line 11, passes through pump 12 and into countercurrent heat exchanger 13 where it picks up heat from out-going condensate .and is raised in temperature to substantially the boiling point. Generally, the amount of condensate available for heating is somewhat less than the amount of sea water to be heated, Iand furthermore not all of the excess enthalpy in the condensate can be recaptured. For these reasons an additional heater 14 may be used if desired to raise the temjerature of the sea water a few additional degrees in its passage through line 16 from the heat exchanger 13 to the atmospheric evaporators 17a-17C. When heater 14 is not used, passage 'of the heated water is through by-pass line 15.

In the `first of the atmospheric evaporators 17a (FIG- URE 2) there are a plurality of first stage evaporating chamber 18a into which the preheated sea 'water is passed. In evaporation chambers 18a, fas will be described more fully hereinbelow, a portion of the water in the brine is evaporated, producing steam which is withdrawn through line 19a and brine concentrate which is withdrawn through line 21a. The steam is compressed by compressor 22a to a relatively small pressure increment above the atmospheric pressure to which it was generated. The compression is Substantially adiabatic and the stream is thus raised to a higher temperature than the temperature at which it was generated, both because the saturation temperature for the steam is higher at the higher pressure 'and because a small amount `of superheat is imparted to the steam. The compressed steam is passed to desuperheater 23a in which a small amount of water is injected into the steam to remove its superheat and to bring it to the condensation temperature at its slightly elevated pressure.

The desuperheated steam is passed into first stage heating chambers 24a which are separated from the evaporating chambers by heat transfer walls 26a. The pressured steam condenses in heating chambers 24a, thereby losing heat content which passes through the heat transfer walls 26a and supplies the heat to evaporate the brine in the evaporator chambers. The condensate produced in the heat exchangers passes out of the chambers through line 27a and to return line 28.

The brine concentrate passing out of the evaporating chambers 18a is split into a recycle portion which passes through line 29a and is blended with the preheated sea water feed and a product portion which passes through line 31a to the second stage atmospheric evaporator 17b. The proportion of recycling condensate to product concentrate may vary considerably, as for example, from about zero to about 20 parts of recycle condensate per part of product concentrate and preferably about 7 to l5 parts of recycle condensate per part of product concentrate.

A second stage atmospheric evaporator 17b operates in a manner similar to first stage atmospheric evaporator 17a with evaporating chambers 18b, steam lines 1917, condensate lines 2lb, compressor 22h, desuperheater 23h, heating chambers 24]), heat transfer walls 26h, condensate lines 27h, recycle lines 2917, and condensate product lines 31b being comparable to the similar elements in the first atmospheric evaporator 17b.

FIGURE 2 shows three atmospheric evaporators, the last being 17C (with its component chambers, lines, walls, pump and compressor being designated on the `drawing similarly to the comparable parts of the evaporator 17a, except with the subscript c). In an actual embodiment it is preferred to use at least four stages of atmospheric evaporation and up to about 20 to 30 stages. In the last stage a sodium chloride recycle stream, from a source hereinafter specified, is blended with the brine concentrate feed. The condensate from atmospheric evaporator 17e passes through line 27C to condensate manifold 28 and thence through heat exchange 13 as hereinbefore stated. After losing its heat in the heat exchanger the condensate is passed through line 32 to condensate storage.

The product concentrate which is passed out of the last atmospheric evaporator through line 31e preferably has had at least 70% of its water removed and most preferably from about 72 to about 76% removed. At this concentration, under the conditions of the invention, there is a considerable amount of precipitated salt in suspension in the brine, almost enough to thicken the brine to the point where wall film flow thereof cannot be maintained.

FIGURES 3 and 4 show a single atmospheric evaporator, such as the first stage evaporator 17a and shows its general structure. FIGURE 3 shows a set of four evaporating chambers 18a and a set of three heating chambers 24a. It is preferred that at least six each of heating and evaporating chambers be supplied for the first atmospheric evaporator 17a with the number decreasing for subsequent evaporators as the volume of liquid decreases.

In the atmospheric evaporator the heated sea water in line 16 is distributed to the several feeder lines 33a to distributors 34a situated above heating chambers 24a and separated therefrom by barriers 36a. The sea water passing into the distributors 34a spills over the serrated edges 37a at the top of walls 38a which are upward extensions of heat transfer walls 26a. As the sea water spills over the serrated edge, it forms a film 35a on the wall extension 38a which flows downwardly over the wall extension and over the heat transfer wall therebelow. The upward extensions 38a are of suflcient height to assure the desired downward velocity of the flowing film by the time it passes the level of barriers 36a and flows onto the heat transfer wall 26a. The height necessary to achieve the desired theoretical velocity may be calculated from the well-known formula wherein v is the desired velocity, vo is the initial velocity, a is the acceleration due to gravity (32 feet per second per second) and s is the vertical height. Since v is usually zero, it may be seen that the minimum Idesired theoretical velocity of 14 feet per second requires that the height of the extension be at least about three feet. Preferably, the extension is about five feet high, producing a theoretical velocity of about 18 feet per second. Of course, the actual velocities achieved are short of these theoretical velocities; but the actual velocities are very difficult to ascertain with any great degree of accuracy and can vary, depending upon the viscosity of the distilland, etc. Hence, these theoretical velocities are presented here merely as an approximation of comparison of what the actual velocities may be for different height wall extensions. However, it is to be understood that regardless of the precise velocities actually achieved, such three'- and five-foot wall extensions have proven to be generally satisfactory in the present invention. In FIGURE 2 the total height is designated as the dimension L and the height of the upward eXtension is designated as the dimension L'.

As the film of sea water progresses down the heat transfer wall 26a, it increases its downward velocity. It is preferred that the total height of heat transfer wall 26a and its upward extension 38a be at least 16 feet so that the theoretical velocity at the bottom of the heat transfer wall shall be at least 32 feet per second. Preferably, the total height is between about 20 and about 24 feet. The flowing film becomes thinner as it flows `down the heat transfer wall both because of its increased velocity and because of the evaporation of a portion of the water therein. The film thickness at the top of the heat transfer wall is generally in the range of about 0.01 to about 0.1 inch in thickness.

The flowing film picks up heat from the heat transfer wall and a portion of the water therein is evaporated. During a single pass over the heat transfer wall a relatively minor proportion of the water content (of the order of about 5 to about 20%, preferably about 10%) is evaporated. The amount of evaporation in a single pass is limited in order to avoid a substantial change in the boiling point of the brine in any one stage. The brine, being more concentrated at the bottom of the heat transfer wall, has a somewhat higher boiling point than at the top. For this reason, it is preferred to concentrate the brine in at least four stages of atmospheric evaporation, and possibly as many as 20 to 30 stages.

The heat transfer Walls are generally flat plates for simplicity of construction. 16 gage aluminum sheet may be used.

The distance between the heat transfer walls is dependent upon the rate of evaporation of the liquid. From sea water about l0 to about 30 lbs. of water per hour are evaporated per square foot of heat transfer wall, and generally about 20 lbs. of water per square foot. At such rate of evaporation, the heat transfer walls are generally about 2 to 3 inches apart.

The vapors produced by evaporation of water from the brine flow are passed out of the evaporation chambers through lines 39a which lead to line 19a. The steam is passed to compressor 22a which adiabatically compresses it to a pressure increment of about l to 5 p.s.i., thereby raising its temperature by about 10 F. to about 50 F. The temperature increment includes a superheat increment of about 7 F. to about 35 F. which is removed in desuperheater 23a by the injection of a small amount of water, usually less than about 1% of the weight of the superheated steam. The desuperheated steam at a temperature between about 3 F. and 15 F. higher than its highest temperature of generation at the bottom of the evaporating chamber is then passed into the heating chambers 24a through headers 41a containing openings 40a. As the saturated steam loses heat to the heat transfer Walls 26a, it condenses on the walls and flows downwardly as a film on each wall. In order to avoid buildup of the condensate films to a thickness which would impede heat transfer therethrough, collectors 42a are provided at spaced Vertical locations on the walls. It is usually desired that the thickness of the condensate film be limited to not more than about 0.005 inch. Spacing of the condensate collectors at a vertical displacement from each other of no more than about four feet, and preferably no more than about two feet, achieves this result.

The condensate collectors 42a slope downwardly, as shown in FIGURE 4 toward the downcomer lines 43a which connect with line 27a and thence to condensate line 28.

Although the temperature differential between heating chamber and the evaporating chamber is quite low, eX- cellent heat transfer is achieved by a combination of several factors, including the limitation of the thickness of the condensate film, the high velocity of the brine film and the avoidance of fouling on the evaporating side of the heat transfer wall `by the low temperature differential and by the fast moving brine film.

The concentrated liquor is removed from the evaporating chambers through lines 44a which lead to line 21a which in turn leads to recycle line 29a and to line 31a which feeds the concentrate to the next set of atmospheric evaporators.

The brine is passed through the desired number of stages with the precipitated salts carried in suspension by the high liquid velocity. The concentration of precipitated salts rises with the evaporation of water from the brine until the brine concentrate approaches a viscosity beyond which it cannot be maintained as a flowing film. For sea water containing about 2.38% of sodium chloride, 0.31% of magnesium chloride, and 0.11% of calcium sulfate, the `brine is concentrated until about 70 to 76 wt. percent of the water has been removed.

At this concentration most of the sodium chloride is still in solution but a substantial amount of calcium sulfate is precipitated.

The brine concentrate leaving the last of the atmospheric evaporators through line 31C is preferably passed through filter 46 prior to being introduced into the first flash chamber 47a. Alternatively, the filter may lbe bypassed by line 48. Calcium sulfate is removed from filter 46 and is passed to slurry mix tank 51, as shown schematically by line 49.

In flash tank 47a a moderate vacuum is applied to the brine concentrate to cool it by lowering its boiling point. The cooling in flash chamber 47a is substantially adiabatic. Constant stirring is maintained in flash chamber 47a by means of stirrer 49a. The brine concentrate from the first flash chamber is removed through line 52a and passed to the second flash chamber 471:. Generally, a vacuum of the order of about 15 inches of mercury is applied in the first vacuum flash chamber. The number of vacuum flash chambers may vary suitably from about two to about four with three being shown in FIGURE l. In the last vacuum flash chamber 47C a reduced pressure of about 20 to 24 inches of mercury is maintained so that the temperature therein is of the order of about 140 to about 160 F. The additional water evaporated from the flash chambers is passed out through lines 53a, 53b, and 53C to header 54 from which it may be combined with other steam generated in the process or may be utilized for heating purposes in the process.

In the flashing chambers the total temperature drop is about 100 F. (from about 250 F. in the last atmospheric evaporating chamber to about 150 F. in the last flash chamber) making about 100 B.t.u. per pound of concentrate available for additional evaporation. This amount of heat will evaporate about of the remaining water in the concentrate passing to the flash chambers.

The brine slurry from the last flash chamber 47C is passed through line 52C to filter 56 from which the calcium sulfate cake is withdrawn to slurry mix tank 51 as shown schematically by line 57 while the filtered concentrate passes through line 58 to the evaporative crystallizer 59. The filtered concentrate is about -20% of the weight of the original sea water.

The precipitated salt in the slurry mix tank from filters 46 and 56 is stirred with condensate water introduced through line 61. The salt is reslurried and passed through line 62 to filter 63 from which calcium sulfate is taken off as indicated by line 64. Filter 63, as well as filters 46 and 56 described above, are preferably continuous rotary filters, such as Oliver filters, with a doctor blade continuously removing the filter cake.

Hot wash water introduced through line 66 selectively redissolves the sodium chloride from the lter cake while precipitating calcium sulfate from the liquor on the cake. The filtrate from filter 63 is recycled to the atmospheric evaporation step, and particularly to the last evaporating chamber 18C through line 67, as shown in FIGURES 1 and 2. The amount of sodium chloride in the recycle stream recovered from lter 63 is generally between about 10 and about 30% of the original sodium chloride content of the sea water.

In the first evaporative crystallizer 59 the pressure is reduced to about 2.6 inches of mercury absolute and a temperature of about F. is maintained. Additional evaporation takes place and small crystals of sodium chloride, called seed crystals are formed in the filtrate. The slurry of seed crystals is passed through line 68 to a second evaporative crystallizer 69 in which a pressure of about 3 inches of mercury absolute and a temperature of about F. are maintained and in which the seed crystals grow while additional evaporation takes place. The slurry then passes through line 71 to the third evaporative crystallizer 72 which serves as a cooling crystallizer in which crystal size is `built up to about 24 mesh. In the cooling crystallizer a pressure of about 2 inches of mercury absolute and a temperature of about 130 F. are maintained. The first and second evaporative crystallizers are heated, as is well known in the art, by continuous withdrawal of a portion of the contents, passage through heat exchangers (not shown) and recycle.

The concentrated slurry from the cooling crystallizer is passed through line 73 to centrifugal separator 74, in which the precipitated sodium chloride is separated from the concentrated liquor which contains sodium chloride and magnesium chloride. The solids recovered on the centrifugal separator are washed with hot water (about F.) introduced through line 75, and the crystals are reduced in size from about 24 mesh to about 30 mesh. The washing removes substantially all of the calcium sulfate from the sodium chloride product. Removal is not effected by dissolution of the calcium sulfate, but rather by physi- `cal transport in the wash water since the calcium sulfate is flocculent while the sodium chloride is relatively dense.

The `washed crystals comprising substantially pure sodium chloride are passed to sodium chloride storage as shown schematically by line 76. The concentrated liquid, called bitterns is passed through line 77 with a portion being recycled through line 78 to the lrst evaporative crystallizer. Generally, the proportion of bitterns recycled is between about 25 percent and about 75 percent of the total bitterns removed from the centrifugal separator. The portion of `bitterns which is not recycled (about 5 percent by weight of original sea water) may be subjected to treatment to recover magnesium salts and other materials therefrom, as is well known in the art, or may be discarded.

The overall process is quite economical with respect to heat input requirements. While the iboiling of 100 pounds of water `by direct heat input requires about 100,000 B.t.u., the method of this invention requires compression power equivalent to only about 5,000 B.t.u. for the same amount of evaporation. The power requirements of the pumping operations art relatively low, of the order of about 10 B.t.u. per 100 pounds of water evaporated.

While the invention has been described above with par ticular reference to the recovery of sodium chloride from sea water, it is to ibe understood that it is generally applicable to the recovery of either solvent or solute in any solution of a solid in a liquid. The invention is useful in the evaporation of brines other than sea water for recovery of either the salt or the water therein, or both. The invention is useful in the evaporation of saline solutions of low sodium chloride content, such as brackish waters, or in the evaporation of saline solutions of high sodium chloride content, such as the waters of salt lakes or inland seas.

FIGURES 5 through 15 illustrate an evaporating unit 100 of a second embodiment of the present invention, especially adapted for the recovery of solvent. As indicated previously, the embodiment illustrated by this unit 100 has been found to be especially effective in evaporating sea water in the initial stages of `the overall evaporating process illustrated in FIGURES 1 and 2, during which stages roughly about half of the water is evaporated. Thus, for example, this unit 100 can well be utilized as the evaporator indicated at 17a in FIGURE 2. Since the operation of the units 17a, b and c have been described previously herein, it is believed that the operation of the 9 apparatus shown in FIGURES through 15 will be readily understood by now describing the construction of the same and indicating only lgenerally the function of its component parts.

This evaporating unit 100 can :be seen to have a general box-like configuration and thus comprises front and rear vertical walls 102 and 104, respectively, and two vertical side walls 106. These walls 102, 104 and 106 are closed by top and bottom covers 108 and 110, respectively, to form a substantially closed, box-like structure. Extending longitudinally between the front and rear walls 102 and 104 and fixedly secured thereto, are a plurality (six in the specific apparatus shown herein) of vertical heat exchange walls 112, these walls being disposed in general parallel relationship with one another and with the side walls 106. The top and bottom edges 114 and 116, respectively, of each of the Walls 112 terminate a moderate distance (eg, about a half foot) short of, respectively, the top and bottom cover plates 8 and 110, so that there is provided in the unit 100 an upper chamber 118 for the infeed apparatus for the distilland which is to be passed down the heat exchange walls 112, and a lower collecting chamber 120 to collect the distilland as it drops from the heat exchange walls 112.

Alternate adjacent pairs of the heat exchange walls 112 are provided with top and bottom horizontal closure plates 122 and 124, respectively, so that these alternate pairs of heat exchange walls 112, with their related plates 122 and 124 and with the portions of the front and rear walls 102 and 104 located therebetween, define closed chambers 126 which function as vapor condensing chambers. The spaces between the remaining pairs of heat exchange walls 112 are open both top and bottom, and thus define charnbers 128 which are in free communication with the aforementioned upper ldistilland intake chamber 118 and the lower `distilland collecting chamber 120. These chambers 128 function as evaporating chambers and are located alternately between the condensing chambers 126, Also, each of the two side walls 106 defines, with its adjacent heat exchange wall 112, a related one of the evaporating chambers 128.

Extending a short distance upwardly (e.g., about two inches) from the top edge 114 of each heat exchange wall 112 is a related one of several ange members 130. Alternate pairs of these flange members 130, with the related plate 122 extending therebetween, define a related one of several distilland distributing troughs 132 (see FIG- URES 10 and 11), each of which is located in the upper distilland intake chamber 118 immediately above a related condensing chamber 126. To distribute distilland to each of these troughs 132, there is provided at the lupper rear end of the unit 100 a distilland intake manifold 134 which receives t-he distilland through a single intake pipe 136 and Ifeeds the distilland into several intake openings 138, each of which is provided at the rear end of a respective one of each of the troughs 132, Each trough 132 has a section of screen 140 located therein to dissipate any turbulence in the distilland flowing from the openings 138 into the troughs 132. This is conveniently accomplished by arranging each section of screen 140 in a roll of one or more layers, the diameter of the roll of screen being approximately the same as the width of its trough 140, and with the roll of screen 140 lying lengthwise in its related trough 132. The distilland Hows in through the intake manifold and through the openings 138- into the several troughs 132, from which the distilland falls in a film down the evaporating surface 142 of each of the heat exchange walls 112. (The evaporating surface 142 is, of course, that wall surface facing its related evaporating chamber 128.) The distilland which falls from the bottom edge 116 of each wall 112 is collected in the lower chamber 120 and passes out a pipe 144 to be further processed in the manner indicated previously herein.

The vapor which evaporates from the falling films of distilland in the evaporating chambers 1218, is withdrawn from the chambers 1218 by means of a vapor collecting manifold 146 (see FIGURES 6, 9 and 15) located at the lower front portion of the evaporating unit 100. This manifold 146 has a general box-like structure; its width is the same as that of the evaporating unit and its height in the particular apparatus shown herein is about one-third to one-quarter that of the entire evaporating unit 100i. So that the manifold 146 can communicate properly with the evaporating chambers 128, those portions of the front wall 102 which lie within the manifold 146 and are adjacent the evaporating chambers 128 are entirely cut away to form vapor outlet openings 148 of a relatively large cross-sectional area. Thus, vapor passes from the chambers 128 through these openings 148 into the manifold 146; and the vapors pass from the manifold 146 through an outlet conduit 150 to a compressor (not shown in FIGURES 5 through 15).

As indicated previously in the general description of the process of the present invention, these vapors are compressed substantially adiabatically, then desuperheated by the addition of water, and then passed into the condensing chambers, which in the present embodiment are designated 126. To feed the compressed vapors into the condensing chambers 126, there is provided at the upper front portion of the unit 100 an infeed manifold 152 (see FIG- URES 5, 6 and 8) having a general box-like configuration. The compressed vapor travels into the manifold 152 through an intake conduit 154, and passes from the manifold 152 into the uppermost portion of the condensing chambers 126 through openings 156 formed in those portions of the front wall 102 which are within the manifold 152 and adjacent the condensing chambers 126.

As previously described herein, the compressed vapors condense in the chambers 126 on the heat exchange wall surfaces 158 (i.e., condensing surfaces) which face the condensing chambers 126.

Condensate traps 160 (see FIGURES 5, 6, l2 and 14) are provided on each of the condensing surfaces 158 at vertically spaced intervals of one foot. As illustrated in FIGURE 12, each condensate trap 160 comprises an -upper flange portion 162 by which it is joined to its related heat exchange wall 112, and a lower trough portion 164 to collect the condensate formed on the heat exchange surface portion 158 thereabove. Each such trap 160 has a moderate rearward slope so that the condensate collected therein flows to the rear of each condensing chamber 126 and falls to the bottom of the condensate chambers 126. A manifold 166 is provided at the lowermost end of the condensing chambers 126 to collect the condensate from the lower end of the chambers 126. This manifold 166 has an outlet pipe 166a and a pressure relief valve, indicated at 166b.

To provide lateral support for the several heat exchange walls 112, there are located in each condensing chamber 126 a plurality of bracing plates 168. Each of these bracing plates 168 is conveniently attached to its two adjacent heat exchange walls 112 by forming the two edge portions of each plate 168 as downturned lips 170 which engage the upturned edge of the related condensate trough 164 (see FIGURE 12). Most of the area of each plate 168 is formed as through openings 172 so that the steam can pass freely down through the condensing chambers 126. Brackets 174 are secured to the top of -the unit 100 so that support from an overhead location can be provided for the unit 100.

While the mode of operation of this unit 100 should be readily apparent from this detailed description of the same, when read in light of the previous description of the overall process of the present invention, it is believed the following comments will be of further aid in understanding the merits of the present invention.

With regard to the vapor collecting manifold 146, the cross-sectional area of the openings 148 should be sufficiently large so that the vapor does not have a very high velocity as it leaves the vaporizing chambers 128. Otherwise the vapor will become contaminated with droplets of the brine. Experimental results indicate that if the velocity of the vapor through the openings 148 does not exceed roughly about two feet per second, relatively pure vapor can be collected. If it is desired to so build the apparatus to extend the heat transfer walls laterally, in order to keep this vapor velocity within the desired limits, the manifold 146 can be extended upwardly; and a second vapor collecting manifold can be provided at the rear of the unit 100.

In the apparatus disclosed in the first embodiment of the present invention, provision was made for imparting an initial downward velocity to the falling film of brine as it comes into contact with the heat transfer surface. As indicated previously, this becomes especially critical when handling brine of higher salt concentrations. However, with lower salt concentrations, it is possible to pass the falling film onto the heat transfer walls with a very low initial velocity and thus utilize substantially all of the wall area for heat transfer.

To examine experimentally the heat transfer characteristics of the heat transfer walls as -used in the present invention, a single heat transfer wall was erected. It was fifteen feet in height and was made of 18-gage aluminum sheeting. On the condensing side of the sheet, seven collecting gutters were provided at two-foot intervals. Each side of the wall was enclosed with a respective box-like structure so as to form an evaporating chamber and a condensing chamber, generally similar to those shown previously herein. Relatively pure water (i.e., tap water, which has essentially the same heat transfer characteristics as sea water which has not been concentrated to an appreciable extent) was raised to 212 F. and passed down the evaporating side of the wall with the pressure in the evaporating chamber being atmospheric. Saturated steam having a pressure of about one pound per square inch gage was fed into the condensing chamber. Vapor was removed from the evaporating chamber, and condensate was collected from each gutter separately by means of a plurality of hoses (each leading from a related gutter) so that the heat transfer characteristics could be determined for each wall portion between a related pair of proximate gutters. A number of runs were made in which the flow rate of water and the temperature differential between the condensing chamber and the evaporating chamber were each varied independently. The results of these runs were tabulated and are summarized in the accompanying graph of FIGURE 16.

In the graph of FIGURE 16, curve A is a plot of the overall heat transfer coefficient for the entire heat transfer wall (measured in terms of B.t.u.s per square foot per hour per degree Fahrenheit temperature differential) when the temperature differential is varied and the ow of water is maintained at a substantially constant flow of 11.2 gallons per minute for each lateral foot of the heat transfer wall. Curve B is a plot of the overall heat transfer coefficient for the entire heat transfer wall when the temperature differential was held substantially constant at three degrees Fahrenheit and the flow of water down the heat transfer wall was varied. In general, it can be seen that the overall heat transfer coefficient was relatively high for temperature differentials between two and four degrees Fahrenheit, and also relatively high for ows which varied between 7.5 and 11.5 gallons per minute per lateral foot of wall. Further, it was found that the heat transfer coefficient for each wall section between each adjacent pair of gutters was high. Experimental results other than those used to draw the graph of FIGURE 16 indicate that heat transfer coefficients yet higher than those indicated in the graph of FIGURE 16 can be attained in practicing the present invention.

To examine further the operating characteristics of the present invention, an evaporating Lmit was constructed substantially identical to that shown in FIGURES through 15. The heat transfer walls 112 were fabricated of IS-gage aluminum sheeting; these were made 152/3 feet long and 21/2 feet wide, and were spaced three inches from one another. The Unit was operated in the manner described previously herein, using fresh water as the distilland. Based on data taken during the operation of this unit, the graph of FIGURE 17 was prepared to show the operating characteristics of the unit. In the graph, the pounds per hour indicated is the amount of water condensed from the unit; the C.F.M. indicates the cubic feet of vapor per minute evaporated from the unit; AT F. indicates the temperature differential in degrees Fahrenheit, and AP PSI indicates the pressure differential in pounds per square inch between the condensing and evaporating chambers. From this data, it is believed that those skilled in the art will be able to select the operating conditions best suited for a particular operation (e.g., in terms of desired output, overall economy in view of power costs, etc.). For example in an area where power is inexpensive relative to the costs of installing the apparatus, it would be better to operate at the upper end of the curves. On the other hand, if opposite conditions prevail, it would bev better to operate at the lower end of the curves.

It is to be understood, of course, that if a unit such as this were to be made for a large commercial operation, the number of heat transfer walls would be increased; and quite possibly the lateral dimension of the heat transfer walls would be increased. The number of stages used would depend on various things, such as whether or not it is desired to recover salt from the brine, etc.

In the unit shown in FIGURES 5 through 15, the condensate collecting gutters were spaced vertically one foot apart. As indicated previously, this arrangement of vertically spaced gutters is considered to be especially significant in the present invention in obtaining a proper heat transfer balance. Perhaps this can be best understood with reference to FIGURE 18, wherein is shown the heat transfer characteristics at specific locations over a sixteenfoot vertical wall where there is a free falling film on one side of the heat transfer wall, and steam condenscs on the opposite side of the wall with no provision for :gutters or other means to remove the condensate at vertically spaced intervals. Thus the condensate which collects on the upper wall portions falls to the lower wall portions.

There is a first curve of the heat transfer resistance caused by the free falling film on the evaporating side of the heat transfer wall. This resistance is highest at the top of the wall and decreases downwardly until it becomes substantially constant over about the lower half of the wall. There is a second curve of the resistance on the condensing side of the wall; and from this curve it is apparent that as condensate collects and flows to the lower VWall portions to increase the condensate film thickness over the lower wall portions, the resistance on the condensing surface at the lower wall 'portions increases until 'it is quite disproportionate to that on the evaporating side of the wall. There is a third plot Awhich is the heat transfer resistance of the aluminum wall. This resistance is relatively small and is constant over the entire wall, and hence appears as a straight line at the lower portion of the graph. The fourth curve (i.e., the uppermost curve) represents the total resistance.

In the present invention, by providing gutters at properly spaced intervals of one or two feet, the heat transfer resistance on the condensate surface is not permitted to become disproportionate to the heat resistance on the opposite wall surface. For example, if the gutters are spaced at two-foot intervals, the heat transfer resistance of the condensate film that occurs within the `rst two feet of the Wall would be essentially the same for each twofoot vertical interval for the entire wall. Obviously, if the gutters were spaced at one-foot intervals, the resistance of the condensate film -would be lower yet for the entire wall surface. Within these l'imits, it becomes a matter of design requirements as to how close to make the spacing of the gutters in accordance with the desired performance of the unit. However, when the spacing of the gutters becomes greater than about four feet, it can be seen that the heat transfer resistance of the condensate film is becoming quite disproportionate so as to destroy the balance of heat transfer coefficients on each side of the wall and thus impair the proper operation of the unit.

It is also to be understood that numerous modifications of the process and apparatus described-above may be made within the scope of the invention. For example, the des'ired downward velocity of the flowing film on the heat transfer surface need not be achieved by an upward extension of the heat transfer wall but can be achieved instead by forming the film from a stream which has a downward velocity imparted to it by a pump. When the film has an initial downward velocity it is especially preferable to have a fiat heat transfer wall since liquid entering a tubular channel at high velocity forms eddy currents which erode and eventually cut through the tube.

The condensate film may be controlled in thickness by means other than the collection troughs described above,

as for example, by weirs spaced from the wall by the de-` sired distance.

It is also to be understood that the number of atmosphereic evaporators, of flashing tanks and evaporative crystallizers and the dimensions thereof may be varied within the general teachings of this invention.

I claim:

1. In an evaporating apparatus, a vertically disposed evaporating chamber, a vertically disposed heating chamber separated from said evaporating chamber by a vertical heat transfer wall, means for introducing a heating vapor into said heating chamber, means for controlling the thickness of a film of condensed heating vapor on said heat transfer wall, means for removing liquid condensate from said heating chamber, means for introducing a liquid to be evaporated to the upper portion of said evaporating chamber, means for removing vapor from said evaporating chamber and means for removing liquid from said evaporating chamber, said liquid introducing means comprising an upward extension of said heat transfer wall to at least about 3 feet above the uppermost heat transfer level, means for introducing said liquid to the upper portion of said wall extension as a film thereon, and said film thickness control means comprising a plurality of condensate traps on said heat transfer wall at spaced vertical intervals from each other.

2. The evaporating apparatus of claim 1 wherein the total height of said heat transfer wall and said extension thereof is at least about 16 feet.

3. In an evaporating apparatus, a vertically disposed evaporating chamber, a vertically disposed heating chamber separated from said evaporating chamber by a vertical heat transfer wall, means for introducing a condensible heating vapor into said heating chamber, means for controlling the thickness of a film of condensed heating vapor on said heat transfer wall, means for removing condensed heating vapor from said heating chamber, means for introducing a liquid to be evaporated to the upper portion of said evaporating chamber as a vertically downwardly moving film on said heat transfer wall, means for removing vapor from said evaporating chamber, and means for removing liquid from said evaporating chamber, said liquid introducing means comprising an upward extension of said heat transfer wall and the combined vertical height of said heat transfer wall and said extension being at least 16 feet.

4. In an evaporating apparatus, a vertically disposed evaporating chamber, a vertically disposed heating chamber separated from said evaporating chamber by a vertical heat transfer wall, means for introducing a heating vapor into said heat chamber, means for controlling the thickness of a film of condensed heating vapor on said transfer wall, means for removing liquid condensate from said heating chamber, mea-ns for introducing a liquid to be evaporated to the upper portion of said evaporating chamber as a vertically downwardly moving film on said heat transfer wall, means for removing vapor from said evaporating chamber and means for removing liquid from said evaporating chamber, said liquid introducing means comprising an upward extension of said heat transfer wall, the combined vertical height of said heat transfer Wall and said extension being at least 16 feet, and said film thickness control means comprising a plurality of condensate traps on said heat transfer wall at spaced vertical intervals from each other.

5. The evaporating apparatus of claim 4 wherein said condensate traps are vertically displaced from each other by no more than about four feet.

6. The evaporating apparatus of claim 3 wherein the combined vertical height of said heat transfer wall and said extension is between about 20 and about 24 feet.

7. The evaporating apparatus of claim 3 wherein a plurality of vertically disposed evaporating chambers are separated from a plurality of fiat vertical heat transfer walls by a plurality of fiat vertical heat transfer walls disposed at a distance of about 2 to 3 inches from each other.

8. A method for evaporating brine which comprises passing said brine downwardly as a fiowing film over a heat transfer wall, passing steam into contact with the opposite side of said heat transfer wall at a temperature between about 3 F. and about 15 F. above the temperature of said flowing film, condensing at least a Iportion of said steam as a film on said opposite side, preventing the buildup of said condensate film to a thickness in excess of about 0.005", maintaining a vertically downward velocity on said brine film at the uppermost heat transfer level of said heat transfer wall at least equivalent to the velocity achieved by a free falling film of said brine on a smooth vertical wall after a fall of about 3 feet and evaporating from said fiowing brine film between about 10 and about 30 pounds of water per hour per square foot of heat transfer wall.

9. A method of evaporating brine which comprises passing said brine downwardly as a owing film over a heat transfer wall, passing said fiowing film vertically downwardly at least about five feet prior to entry onto said heat transfer wall, evaporating into an evaporation zone from said flowing brine film between about l0 and about 30 pounds of water per hour :per square foot of heat transfer wall, removing steam from said evaporation zone, cornpressing said steam by a pressure increment of about 1-5 p.s.i. to impart superheat thereto, desuperheating said compressed steam to the saturation point while maintaining substantially all of said pressure increment, passing said compressed steam into contact with the opposite side of said heat transfer wall, condensing at least a portion of said steam as a downflowing film on said opposite side of said heat transfer wall and collecting at least a portion of the liquid condensate in said last named film at vertically spaced locations in its downward path.

10. A method for evaporating a liquid which comprises passing said liquid downwardly as owing film over ah heat transfer wall, said film having a vertically downward velocity at the bottom of said heat transfer wall at least equivalent to the velocity achieved by a free falling film of said liquid on a smooth vertical surface after a fall of 16 feet, passing a heated vapor into contact with the opposite side of said heat transfer wall, condensing at least a portion of said heating vapor as a film on said opposite side of said heat transfer wall and preventing the buildup of said condensate film to a thickness in excess of about 0.005 inches.

11. A method for evaporating a liquid which com- 15 prises passing said vertically downwardly as a flowing film over a heat transfer wall, said lm having a vertically downward velocity at the bottom of said heat transfer wall at least equivalent to the Velocity achieved by a free falling, lrn of said liquid on a smooth vertical surface after a fall of 16 feet, passing a heated vapor into contact with the opposite side of said heat transfer wall, condensing at 'least a portion of said heating vapor as a lm on said opposite side of said heat transfer wall and collecting at least cations in its downward path.

References Cited UNITED STATES PATENTS Smith 202-236 X Lustenader 203-21 X Hickman 202--236 Neugebauer 202-236 Neugebauer 202-236 X NORMAN YUDKOFF, Primary Examiner, a portlon of the liquid 1n said lm at vert1cally spaced l0- 10 D* EDWARDS, Assistant Examiner. 

8. A METHOD FOR EVAPORATING BRINE WHICH COMPRISES PASSING SAID BRINE DOWNWARDLY AS A FLOWING FILM OVER A HEAT TRANSFER WALL, PASSING STEAM INTO CONTACT WITH THE OPPOSITE SIDE OF SAID HEAT TRANSFER WALL AT A TEMPERATURE BETWEEN ABOUT 3*F. AND ABOUT 15*F. ABOVE THE TEMPERATURE OF SAID FLOWING FILM, CONDENSING AT LEAST A PORTION OF SAID STREAM AS A FILM ON SAID OPPOSITE SIDE, PREVENTING THE BUILDUP OF SAID CONDENSATE FILM TO A THICKNESS IN EXCESS OF ABOUT 0.005'''', MAINTAINING A VERTICALLY DOWNWARD VELOCITY ON SAID BRINE FILM AT THE UPPERMOST HEAT TRANSFER LEVEL OF SAID HEAT TRANSFER WALL AT LEAST EQUIVALENT TO THE VELOCITY ACHIEVED BY A FREE FALLING FILM OF SAID BRINE ON A SMOOTH VERTICAL WALL AFTER A FALL OF ABOUT 3 FEET AND EVAPORATING FROM SAID FLOWING BRINE FILM BETWEEN ABOUT 10 AND ABOUT 30 POUNDS OF WATER PER HOUR PER SQUARE FOOT OF HEAT TRANSFER WALL. 